High frequency low pass filter



July 15, 1969 P. A. DENES 3,456,215

HIGH FREQUENCY LOW PASS FILTER Filed Sept. 2, 1964 I 2 Sheets-Sheet 1 l2 '3 r F- 4 1011i 13b 10 20 so 100 200 500 I000 2000 INVENTOR FREQUENCY MC PETER A. DENES July 15, 1969 P. A. DENES 3,456,215

HIGH FREQUENCY LOW PASS FILTER Filed Sept. 2, 1964 2 Sheets-Sheet 2 INvEN-rOR PETER A. D E

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United States Patent 3,456,215 HIGH FREQUENCY LOW PASS FILTER Peter A. Denes, 9101 Crestwood Ave. NE., Albuquerque, N. Mex. 87112 Filed Sept. 2, 1964, Ser. No. 393,946 Int. Cl. HOlh 7/14; H03m 7/10; H01p 1/22 U.S. Cl. 333-49 7 Claims ABSTRACT OF THE DISCLOSURE A low pass filter is disclosed comprising a tube of insulating material constituting a dielectric and a support for at least two axially disposed capacitor sections and an inductance, disposed within the tube. The capacitor sections are formed by a common plate-forming electrode spaced apart from at least two axially spaced capacitor plate-forming electrodes embedded Within the tube of insulating material. The inductive element employs a ferromagnetic material.

This invention relates to pi-type high frequency low pass filters.

Pi-type filters generally utilize an inductance compris ing a number of windings forming a core which at the higher frequencies are shunted by a low impedance formed by the interwinding capacitance of the inductance. This low shunting impedance destroys or greatly reduces the filtering ability of the filters. Some attempts have been made to overcome this problem. For example, a pi-type filter has been made with an inductive element formed by a central conductor or terminal surrounded by a bead or core of ferrite material, the central conductor and the surrounding ferrite bead acting as a single winding which eliminates inter-winding capacitance. However, this filter has two disadvantages. First, the permeability of ferrites is very dependent on the direct current magnetization, and, due to the relatively low saturation induction of ferrites, a small direct or low frequency current through the central terminal can lower the attenuation considerably. Furthermore, the ferrites, being high resistive oxide materials and having medium high frequency losses, represent at high frequencies still a relatively high resistance value equivalent to their losses, and can therefore only partially eliminate self resonance efi'ects inherent in the filter.

An object of the present invention is to provide a filter unit which has the advantages of the ferrite filter unit without its disadvantages so that it is practically independent of the direct current or low frequency current flowing through the central conductor or terminal. A related object of the invention is to provide a filter unit as described, the attenuation of which diminishes only negligibly at internal resonance frequency ranges.

A further object of the invention is to provide a filter unit as described which is exceedingly compact.

In the most preferred form of the present invention, the filter unit comprises a ceramic tube bearing two or more capacitor sections each comprising inner and outer plate-forming electrodes, and an inductor section comprising a central conductor or terminal electrically connected to the inside electrode of each capacitor section, and a ferromagnetic bead surrounding the central terminal. The ferromagnetic inductive section of the filter has no windings except the central conductor or terminal representing one turn of an inductance. The use of a ferromagnetic material for the head of this filter unit has several advantages over the filters containing ferrite cores. Among these advantages is that a filter made with such a "ice head possesses a much higher (as much as five to eight times higher) magnetic saturation induction than prior filters containing ferrite cores and, therefore, can be loaded with a much higher direct current or low frequency current through the central conductor or terminal without appreciably changing the permeability thereof.

The ferromagnetic beads have a permeability which does not vary much within wide variations of the magnetizing field. Also the loss performance of the filter can be varied within very broad limits, and can surpass with orders of magnitude many times that obtainable by ferrite filters heretofore made. Hence, filters according to the present invention can have lossy, ferromagnetic inductive elements which practically eliminate high frequency attenuation drops caused by self-resonances inherent in the elements of the filter.

Another advantage of the ferromagnetic beads over the ferrite heads is their thermal behavior. The ferromagnetic beads have a Curie temperature much higher than that of ferrite beads, and hence, their permeability changes less with temperature. Also, the thermal conductivity of the ferromagnetic metals is much higher than the thermal conductivity of ferrites. Consequently, a much larger high frequency energy can be filtered out with filters of a given size using ferromagnetic beads, because the generated heat distributes more evenly without the danger of cansing cracks as occurs in ferrite beads which are ceramic and very sensitive to thermal stresses.

The ferromagnetic cores used in the filter unit of the present invention may, for example, be windings of a thin ferromagnetic metal tape with insulation between windings, or they may be cores of powdered ferromagnetic metals with an insulating binder or they may be insulated laminations of ferromagnetic metal. The size of the powder or the thickness of the tape or lamination determines mainly the eddy current losses which may yield Q factors at high frequencies as low as 10- or less.

Other features of the invention deal with the manner in which the ceramic tube and capacitor-forming electrodes of the tube are arranged to provide optimum capacity and tube wall strength.

The above and other objects, advantages and features of the invention will become apparent upon making reference to the specification to follow, the claims and drawings wherein:

FIG. 1 is a longitudinal sectional view of a single section pi filter unit illustrating one form of the present invention;

FIG. 2 shows the equivalent circuit of the filter unit of FIG. 1;

FIG. 3 is a longitudinal sectional view of a double section pi filter unit illustrating another form of the present invention;

FIG. 4 shows the equivalent circuit of the filter unit of FIG. 3;

FIG. 5 shows the attenuation curves of various filter structures to show a comparison of the attenuation curves of the prior art filter units and the two forms of filter units of the present invention shown in FIGS. 1 and 3;

FIG. 6 is a longitudinal sectional view of a single section pi filter unit illustrating a preferred form of the present invention;

FIG. 7 is a longitudinal sectional view of a double sction pi filter illustrating still another form of the present invention;

FIG. 8 shows an equivalent circuit of the filter unit of FIG. 7;

FIG. 9 is a longitudinal section through a pi filter unit illustrating a preferred modified form of the filter unit shown in FIG. 7; and

FIGS. 10, 11 and 12 respectively illustrate three forms of ferromagnetic beads usable in the various forms of the invention of FIGS. 1-9.

Referring now more particularly to FIG. 1, the filter unit there shown comprises an open ended ceramic insulator tube 11 having a centered outer conductive cylindrical electrode 12 coated or otherwise placed upon the outer surface of the ceramic tube. The electrode 12 preferably occupies an appreciable portion of the length of the ceramic tube with the outer ends thereof spaced from the outer ends of the tube.

The filter unit further has a pair of inner conductive cylindrical electrodes 13 and 14 having inner cylindrical portions 13a and 14a coated or otherwise applied to the inner surface of the tube 11. The confronting margins of the inner cylindrical portions 13a and 14a of the electrodes 13 and 14 are spaced from one another and underlap the end portions of the cylindrical electrode 12.

The outer electrode 12 forms a common plate for a pair of capacitors having individual plates formed by the cylindrical portions 13a and 14a of the electrodes 13 and 14 and a dielectric constituted by the material forming the ceramic tube 11. The common outer electrode 12 is grounded or connected to a common point in the filter circuit.

The inner electrodes 13 and 14 also have end portions 13b and 14b respectively covering the outwardly facing edges of the ceramic tube 11. In the illustrated embodiment of the invention, the edge covering portions 13b and 14b of the electrodes 13 and 14 extend around to the outer surface of the tube 11, but this is not an important feature of the design of the electrodes 13 and 14.

The inductance portion of the filter unit 16 is formed by a ferromagnetic core or head 19 and a central conductor 16 which passes centrally through the filter unit. This central conductor is fixed in position within the filter unit and electrically connected to the capacitor plateforming portions of electrodes 13 and 14 by conductive washers or discs 17 and 18.

Discs 17 and 18 have central openings 17a-18a through which the central conductor 16 passes and the central conductor 16 is soldered or otherwise physically and electrically connected to the discs. The discs 17 and 18 are physically and electrically anchored to the inner electrodes 13 and 14 preferably by soldering the same to the edge covering portions 132; and 14b thereof. The opposite ends of the central conductor 16 form respectively input and output terminals for the filter unit.

The ferromagnetic core or bead may, for example, be a nickel-iron alloyed powder held together by a suitable non-magnetic binder. In one filter unit, the bead 19 comprised nickel and iron powder in equal amounts, the powder having an average particle size of about microns. FIG. 10 illustrates a head 19 made of a powdered core. The non-magnetic binder was a suitable epoxy resin. Other ferromagnetic materials from which the ferromagnetic core or head may be made are iron and alloys of iron-cobalt, iron-nickel, iron-cobalt-nickel, ironnickel-molybdenum, iron-aluminum, iron silicon-alumi num, iron-nickel-chromium and iron-silicon with minor additions of other materials. The Q factor of the ferromagnetic bead 19 may, for example, be 10" or less at high frequencies above 100 megacycles per second. The desired loss characteristic of the head is varied by varying the size of the powder, the larger the powder granules the greater the eddy current losses.

The ferromagnetic materials referred to, instead of being in powdered form, may be in tape or laminated form as illustrated respectively in FIGS. 11 and 12. As shown in FIG. 11, the head 19 comprises a strip of tape 20 wound about the conductor 16, the layers of tape being separated and bound together into an integral unit by an insulating binder 21. As shown in FIG. 12, the bead 19 comprises a stack of annular rings 20' of ferromagnetic material separated and bound together by an insulating binder 21. The thickness of the windings or lamination referred to can, for example, be of the order of from 2 to microns.

FIG. 2 shows the equivalent circuit for the filter unit 10 just described with the reference numeral applied thereto identifying the corresponding'elements in FIG. 1.

FIG. 3 shows a two section pi filter unit 10a which is similar in many respects to the filter unit shown in FIG. 1. In FIG. 3, the filter unit may have the same tube 11, central conductor 16, outer electrode 12, and discs 17 and 18 shown in the embodiment of FIG. 1. However, the inner electrode and core structure are modified to form the double section pi filter. To this end, a pair of ferromagnetic cores or beads 19a-19b are applied to the central electrode 16 at axially spaced points thereof within the tube 11. A relatively narrow, cylindrical capacitor-forming electrode 24 is coated or otherwise placed on the inner surface of the tube 11 along a narrow band in the central region thereof. Inner electrodes 13' and 14' are coated or otherwise placed on the inner surface of the tube 11 along a narrow band in the central region thereof, the electrode 13' and 14' having capacitor plate-forming portions 13a and 14a located within the end portions of the tube 11. The inner margins of the electrode portions and 14a of the electrode 13' and 14' are spaced from the adjacent ends of the central inner electrode 24 and underlap the outer ends of the outer electrode 12 of the filter unit. The electrodes 13 and 14 also have end portions 13]) and 1411 which cover the outer edges of the tube 11 and are soldered to the end discs 17 and 18.

The central inner electrode 24 is electrically connected to the center portion of the central conductor 16 between the ferromagnetic beads or cores 19a and 19b by a suitable disc or Washer 28. A two-section pi filter is thus formed -where one section comprises the inductance formed by the core or bead 19a and the central conductor 16 in the two capacitors formed by the electrode pair 13a'-12 and 24-12 (see FIG. 4). The other section of the pi filter unit comprises the inductance formed by the core 19b and the central conductor 16 and the capacitor formed by the electrode pair 14a'-12. The electrical equivalent circuit of the two-section pi filter unit of FIG. 3 is shown in FIG. 4.

In the described filter, the capacitive sections have small parallel inductances and the inductive sections small series capacitances. The spurious conjugatory impedances could cause dips in the attenuation values at the resonance frequencies. However, such resonance frequencies are very high due to the very small values of the spurious conjugatory impedances, at which high frequencies the Q values of the ferromagnetic cores are so small that they are acting as resistive elements and eliminate the resonance effects almost completely. Of equal importance, the permeability of the ferromagnetic cores do not vary much with the direct current magnetization of the filter unit and, unlike ferrite filter units, have very little drop-off of attenuation due to the flow of a large direct current or low frequency currents through the filter unit.

To illustrate the advantages of the present invention, reference should now be made to the various curves of FIG. 5. Curve C1 shows the high attenuation at the upper frequency levels for a filter unit which uses an inductor of conventional design comprising helical windings. Fig. C2 illustrates the attenuation curve of a filter unit made with ferrite beads as the inductive element. It should be noted that even the curve C2 shows a significant resonant drop-01f of the attenuation at the upper frequency levels of the curve.

Curves C3 and C4 illustrate, respectively, the attenuation curves for the forms of the invention shown in FIGS. 1 and 3 where the resonant elfects are practically nonexistent due to the eddy current loss in the ferromagnetic beads. Curves C3 and C4 remain practically unchanged even when a high D.C. magnetizing current fiows through the central conductor 16 which create a magnetizing field of 20 oersteds in the ferromagnetic beads 19. Such magnetizing fields saturate most ferrite materials and their permeabilities drop greatly. Curve C2 indicates the attenuation of the ferrite bead filter if a magnetizing field of 20 oersteds is applied on the beads and the drop of curve C2 is as high as 30 db at some frequencies.

If a double section pi filter would be made with ferrite beads, the drop of attenuation caused by a magnetizing field would be generally twice as much in db as the drop between the curves C2 and C2 and would make such filters impractical. The double section pi filters made with ferromagnetic beads have higher attenuation remaining unchanged at magnetizing fields of 20 oersteds.

Explanation of the electromagnetic and physical bases of the difference in behavior of the ferrite and ferro-magnetic cores demonstrated by FIGURE 5 is easily found in a simplified analysis of the following equation:

& L g o QLQo wherein A is the approximately attenuation of a pi filter having one series inductor and a parallel capacitor; X is the reactance of the inductor; X is the reactance of the capacitor; and Q and Q are the respective quality factors. It is obvious that A will increase if either X increases or Q, decreases. For a given size core at a given frequency, X is proportional to the permeability of the core. It is found that if current is flowing through the central conductor 16, ferrite cores saturate easily with an attendant permeability decrease. Consequently, X decreases in turn reducing attenuation. Ferro-magnetic cores, however, have much higher saturation inductions and their permeability is stable up to very high currents flowing through the conductor. Thus, the ferro-magnetic core produces higher attenuation. Further, it has been found that the Q, factor of ferro-magnetic powder cores is much lower than that of ferrites due to the high eddy current losses in the metallic particles of the former. Thus, the attenuation of filters containing powder cores runs higher at high frequencies than those filters having ferrite cores.

Another aspect of the present invention deals with the construction and arrangement of the ceramic tube and the capacitor-forming electrodes thereon in such a way that a maximum capacitance is achieved without sacrificing the strength of the ceramic tube. The thickness of th ceramic tube is most desirably determined solely by mechanical considerations in that it should not distort during a sintering operation and it should withstand the stresses transferred from the central conductor 16 through the end A=20 lo washers 17 and 18 during handling of the filter unit. In I the form of the invention shown in FIGS. 1 and 3, the thickness of the ceramic tube also determines the value of the capacitance so that an increase in the thickness of the tube to satisfy strength considerations would result in a reduction in the capacity and a resultant reduction in the attenuation provided by the filter involved.

In the form of the invention illustrated in FIG. 6, the wall thickness of the ceramic tube 31 does not limit the obtainable capacitance in the filter unit 30. The filter unit has a central conductor 16, a ferromagnetic bead 19 and end discs 17 and 18 which are substantially identical to that shown in the embodiment of FIG. 1. The ceramic tube 31 is substantially thicker than the tube 11 shown in FIG. 1 and the capacitor forming electrodes 33 and 34 thereof have inner cylindrical capacitor plate-forming portions 33a embedded within the body of the ceramic tube 31, with the spacing between these inner capacitor plate-forming portions 33a and 33b from the outer electrode 12 being determined by the desired capacitance of the filter unit. The thickness of the tube 31, as above indicated, is determined primarily by strength considerations.

The ceramic tube and inner electrode structure may be made by using a clipping process as disclosed in US. Patent No. 3,016,597. The inner portion of the tube 31 is built-up by a series of dipping operations which builds-up a coating of a ceramic slurry on a rod to a thickness equal to the distance between inside of the tube and the inner surfaces of the capacitor plate-forming electrode portions 33:: and 33b. The electrode portions 33a and 331: are then painted on by applying spaced noble metal coatings over the ceramic material which extends to the ends of the tube structure, and finishing the ceramic tube by subsequent dipping operations. The resulting tube is then fired in the usual manner to sinter the ceramic mix. A mono- Iithic ceramic tube structure results with the inner electrode portions 33a and 33b imbedded within the tube and extending to the ends thereof as indicated in FIG. 6. The next step in completing the filter unit is to apply conductive coatings over the completed tube to form the outer electrode 12 and the side portions 33b and 34b of the electrodes 33 and 34 which latter portions make physical and electrical contact with the capacitor plate-forming portions 33a and 33]) extending to the edges of the tube. The central electrode 16 carrying the ferromagnetic bead 19 and the discs 17 and 18 are then applied and soldered in place to complete the filter unit.

It should be noted that the type of filter unit illustrated in FIG. 6 is primarily suited for a one-section pi filter unit when the process above described is utilized in building the filter unit. This is so because it is difiicult to make an electrical connection between the central conductor 16 and a central inner electrode completely embedded within the ceramic tube 31.

A still further form of the present invention is illustrated in FIG. 7. The filter unit there illustrated and identified by the reference numeral 10" resembles the form of the invention shown in FIGS. 1 and 2 in that all the electrodes are applied to exposed surfaces of the ceramic tube 11. The filter unit 10 has the same ceramic tube 11, outer electrode 12, central inner electrode 24, inner electrodes 13' and 14', and end discs 17 and 18 shown and described in connection with the embodiment of FIG. 3 which is a two-section pi filter. However, in the form of the invention shown in FIG. 7, there is no electrical connection made between the center inner electrode 24 and the central conductor 16. Thus, as illustrated, the ferromagnetic bead 19a surrounding the center conductor 16 may conveniently extend for substantially the full length of the portion of the central conductor 16 located within the ceramic tube 11. The three capacitors formed by the electrode pairs 13'12, 24-12, and 14'12 are interconnected by annular resistance-forming electrodes 35 and 37 coated on the inside of the tube 11 and bridging the space between the central electrode 24 and the adjacent ends of the electrode portions 13a and 14a.

The equivalent electrical circuit for the filter unit of FIG. 7 is shown in FIG. 8, and, although somewhat different in circuitry from the double section pi filter circuit of FIG. 4, provides a highly satisfactory low-pass filter unit. As shown in FIG. 8, the various capacitors of the filter unit are interconnected by resistance elements 35 and 37 and the inductance formed by the central electrode 16 is connected between the outermost capacitors of the circuit.

The type of filter circuit illustrated in FIG. 8 is very adaptable to the electrode-imbedded form of the invention previously described in FIG. 6 and reference should now be made to FIG. 9 which shows a filter unit of the electrode-embedded type which has the equivalent circuit shown in FIG. 8. The filter unit of FIG. 9 indicated generally by reference 30 is similar to the filter unit of FIG. 7 in that it has the same or similar inductanceforming assembly comprised by the central conductor 16 and the relatively long ferromagnetic bead or core 19a, the end discs 17 and 18 and the outer electrode 12. However, the various inner electrodes corresponding to the electrodes 13', 14' and 24 and the resistance-forming electrodes 35 and 37 in the embodiment of FIG. 7 are embedded within a ceramic tube 31 to form the central inner electrode 24, the resistance electrodes 3537' and the 7 outermost inner electrodes 63' and 34'. The outermost inner electrodes 33 and 34' are constructed in a manner substantially similar to that shown in FIG. 6' and thus include inner capacitor-forming portions 33a and 34a embedded within the ceramic tube 31 and outer electrode portions 33b and 34b.

The ceramic tube 31' and the various electrodes embedded therein may be made in the same manner as described above in connection with the embodiment of FIG. 6. Thus, a raw ceramic mix is initially built-up to the required thickness by a dipping process, the thickness equaling the distance between the innermost surface of the ceramic tube 31' and the innermost surfaces of the embedded electrodes. The various electrodes or electrode portions 33a, 35, 24', 34a. and 37 are then painted on the raw tube following which a further dipping process is carried out to build-up the ceramic tube to the desired thickness. Then, following a firing operation, the other electrodes or electrode portions 12, 33b, 34b, and the other parts of the filter unit described are applied and assembled in the same manner previously described in connection with the embodiment of FIG. 6.

It is apparent that the various forms of the present invention provide a much improved filter unit both from an electrical and mechanical standpoint. It should be understood that numerous modifications may be made in the most preferred forms of the invention described above without deviating from the broader aspects of the invention. In this connection, although the use of ferromagnetic beads around the central conductors described constitute an important aspect of the present invention, some of the unique structural forms of the invention, such as the ones shown in FIGS. 6, 7, and 9, are also adaptable to filter units having somewhat different inductance forming elements than that described.

I claim:

1. A low pass filter for blocking high frequency interference above approximately megacycles comprising a tube of insulating material carrying capacitor-forming electrodes forming at least two capacitor sections, and inductor-forming means comprising a conductor extending through said insulating tube and a core of ferromagnetic material on said conductor for high frequency attentuation, said ferromagnetic core being a metallic powder bound together by an electrically insulating binder, sald conductor being electrically connected at points on opposite sides of said ferromagnetic core respectively to one of the electrodes of each of said capacitor sections, the opposite ends of said conductor respectively constituting input and output terminals to the filter, and a common terminal connected to the capacitor sections at a point remote from the electrodes connected to said conductor whereby high values of direct current or low frequency current can be applied through said conductor without appreciably changing the permeability of said core and whereby the attenuation of said filter is substantially unchanged at internal resonance frequency ranges.

2. A low pass filter for blocking high frequency interference above approximately 10 megacycles compris ng a tube of insulating material carrying capacitor-formlng electrodes forming at least two capacitor sections, and inductor-forming means comprising a conductor extending through said insulating tube and a core of ferromagnetic material on said conductor for high frequency attenuation, said ferromagnetic core being a metallic wound tape the windings being insulated and bound together by an electrically insulated binder, said conductor being electrically connected at points on opposite sides of said ferromagnetic core respectively to one of the electrodes of each of said capacitor sections, the opposite ends of said conductor respectively constituting input and output terminals to the filter, and a common terminal connected to the capacitor sections at a point remote from the electrodes connected to said conductor whereby high values of direct current or low frequency current can be applied through said conductor without appreciably changing the permeability of said core and whereby the attenuation of said filter is substantially unchanged at internal resonance fre quency ranges.

3. A low pass filter for blocking high frequency interference above approximately 10 megacycles comprising a tube of insulating material carrying capacitor-forming electrodes forming at least two capacitor sections, and inductor-forming means comprising a conductor extending through said insulating tube and a core of ferromagnetic material on said conductor for high frequency attenuation, said ferromagnetic core being a metallic laminated core the laminae being insulated and bound together by an electrically insulating binder, said conductor being electrically connected at points on opposite sides of said ferromagnetic core respectively to one of the electrodes of each of said capacitor sections, the opposite ends of said conductor respectively constituting input and output terminals to the filter, and a common terminal connected to the capacitor sections at a point remote from the electrodes connected to said conductor whereby high values of direct current or low frequency current can be applied through said conductor without appreciably changing the permeability of said core and whereby the attenuation of said filter is substantially unchanged at internal resonance frequency ranges.

4. A low pass filter for blocking high frequency interference above approximately 10 megacycles comprising a tube of insulating material carrying capacitor-forming electrodes forming at least two capacitor sections, and inductor-forming means comprising a conductor extending through said insulating tube and a core of ferromagnetic material on said conductor for high frequency attenuation, said ferromagnetic core comprises at least one of the group consisting of iron, nickel, cobalt, and an alloy of at least one of the aforementioned metals, said conductor being electrically connected at points on opposite sides of said ferromagnetic core respectively to one of the electrodes of each of said capacitor sections, the opposite ends of said conductor respectively constituting input and output terminals to the filter, and a common terminal connected to the capacitor sections at a point remote from the electrodes connected to said conductor whereby high values of direct current or low frequency current can be applied through said conductor without appreciably changing the permeability of said core and whereby the attenuation of said filter is substantially unchanged at internal resonance frequency ranges.

5. The low pass filter of claim 4 wherein said alloy is one of the group consisting of iron-nickel, iron-cobalt iron-cobalt-nickel, iron-nickel-molybdenum, iron-nickelchromium, iron-silicon, iron-aluminum and iron-siliconaluminum.

6. A low pass filter comprising a tube of insulating material constituting a support for capacitor-forming electrodes, said tube having a common electrode forming a capacitor plate and terminal, a pair of axially spaced capacitor plate-forming electrodes forming with said common electrode and the tube a pair of capacitors, a resistance-forming electrode bridging the space between said pair of capacitor plate-forming electrodes and constituting a resistance element interconnecting said pair of capacitor plate-forming electrodes, and an inductance within said tube coupled between the outermost portions of said pair of capacitor plate-forming electrodes, wherein said capacitor plate-forming electrodes and said resistanceforming electrodes are embedded within said tube.

7. A low pass filter comprising a tube of insulating material constituting a support for capacitor-forming electrodes, said tube having a common electrode forming a capacitor plate and terminal, at least three axially spaced capacitor plate-forming electrodes forming with said common electrode and the tube at least three capacitors, reslstance-forming electrodes bridging the spaces between said capacitor plate-forming electrodes and constituting resistance elements interconnecting said capacitor plateforrning electrodes, and an inductance within said tube coupled between the outermost portions of the outermost capacitor plate-forming electrodes, wherein said capacitor plate-forming electrodes and said resistance-forming electrode are embedded within said tube.

References Cited UNITED STATES PATENTS 2,228,797 1/1941 Wasserman 17845 3,035,237 5/1962 Schlicke 33379 3,275,954 9/1966 Coda 333-79 Garstang 33379 Garstang 33379 Dahlen 33379 Schlicke 333-79 Schlicke et a1. 333-79 Cook 33381 Weber 333-22 HERMAN KARL SAALBACH, Primary Examiner 10 C. BARAFF, Assistant Examiner US. Cl X.R. 

